1. Field of the Invention
The present invention relates to a combustion control apparatus for an engine and more particularly to a combustion control apparatus capable of switching a combustion mode over from a spark ignition combustion to a compression ignition combustion and vice versa according to engine operating conditions.
2. Discussion of Background Arts
Recent engines realize ultra lean mixture by charge stratification, intensive air turbulence and the like using in-cylinder injection type fuel injectors for directly injecting fuel into cylinders. These engines can operate with a wide range of air-fuel ratio by changing over between an operation with lean air-fuel mixture aiming at a better fuel economy and an operation with stoichiometric air-fuel ratio aiming at an improvement of air utilization ratio.
In general, when an engine operates in a condition of stoichiometric air-fuel ratio, hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in exhaust gas are purified by oxidation and reduction of a three-way catalyst. On the other hand, since a normal three-way catalyst has an inadequate purification capability of NOx due to the effect of excess oxidation under a condition of lean air-fuel ratio, a lean NOx catalyst capable of reducing NOx in an atmosphere of excess oxidation or a NOx occlusion catalyst for reducing NOx in a condition of rich air-fuel ratio or at an area of stoichiometric air-fuel ratio, are used in combination with the three-way catalyst (see Japanese Patent Application Laid-open No. Toku-Kai 2000-337130).
However, the lean NOx catalyst has a defect of small purification efficiency and its emission performance is insufficient. Further, since sulfur contained in fuel or lubrication oil is emitted as sulfuric oxides in exhaust gas, NOx reduction catalysts such as lean NOx catalyst and NOx occlusion catalyst are poisoned by these sulfuric oxides. As a result, the durability of catalyst is much more inferior to that of the three-way catalyst.
It is an object of the present invention to provide a combustion control apparatus capable of reducing NOx emissions under a condition of ultra-lean air fuel ratio without using NOx reduction catalysts and to provide a combustion control apparatus excellent in durability and reliability.
To attain the object, a combustion control apparatus for an engine having a three-way catalyst and a variable valve timing mechanism capable of varying valve opening and closing timings of intake and exhaust valves comprises an air fuel ratio zone establishing means for establishing an air fuel ratio zone to an ultra lean zone where an air fuel ratio is ultra lean, when the engine operates in a low and medium load area and in a low and medium speed area and for establishing an air fuel ratio zone to a stoichiometric zone where an air fuel ratio is stoichiometric, when the engine operates in other load areas and in other speed areas, and a combustion mode establishing means for establishing a combustion mode to a compression ignition combustion mode where a compression ignition combustion is executed, when the air fuel ratio zone is established to the ultra lean zone, and for establishing the combustion mode to a spark ignition combustion mode where a spark ignition combustion is executed, when the air fuel ratio zone is established to the stoichiometric zone. The spark ignition combustion is performed in accordance with a feedback control of air-fuel ratio while the three-way catalyst functions as a three-way catalyst and the compression ignition combustion is performed in accordance with a non-feedback control of air-fuel ratio while the three-way catalyst functions as an oxidation catalyst.